DESCRIPTION: The identification of co-receptors required for HIV infection has raised the question of the respective roles of CD4 and the co-receptors in binding, fusion, and infection. Recent reports of HIV variants that can infect and/or fuse with CD4-negative target cells that express the CXCR4 co-receptor suggest that CD4 may induce conformational changes in the gpl20 envelope glycoprotein that expose the co-receptor binding site, and that the envelope-co-receptor interaction actually mediates the fusion process. To explore this possibility further, the investigators have derived a variant of HIV-1/IIIB, termed IIIBx, that is able to fuse with and infect CD4-negative cell lines, using CXCR4 as its primary receptor. They hypothesize that mutations in the envelope glycoprotein of this variant have resulted in an increased exposure and/or an enhanced affinity of the CXCR4 binding site on gpl20. They have derived a functional envelope clone of IIIBx that maintains the CD4-independent phenotype, and sequence analysis has implicated a number of novel mutations in both gpl20 and gp41 that could be involved in this phenotype. The availability of a large number of monoclonal antibodies to the HIV-1/IIIB envelope glycoprotein, the recently determined crystallographic analysis of a modified gpl20/CD4 complex, and an extensive literature describing the mutagenesis of this protein will facilitate comparative studies between HIV-1/IIIB and the CD4-independent IIIBx variant. Further studies of this glycoprotein should yield valuable insights into the nature of the envelope-CXCR4 interaction and the role of this interaction in the fusion process. Moreover, the envelope glycoprotein of this variant may be useful as an immunogen to produce novel antibodies to sites on gpl20 or gp41 that are involved in post-CD4-binding events. In this proposal the investigators will: (1) map the mutations responsible for the CD4-independent phenotype of IIIBx; (2) identify changes in the structure and function of the IIIBx envelope glycoprotein involved in CD4 independence using antibody cross-competition analysis, biological assays to assess the sensitivity of IIIBx to neutralizing agents, and direct binding assays with recombinant IIIBx envelope and CXCR4; and (3) evaluate the ability of the IIIBx envelope glycoprotein to elicit novel neutralizing polyclonal and monoclonal antibodies. These studies should further the understanding of CD4-induced conformational changes that are relevant to viral entry and provide insights on how these changes can be exploited to neutralize HIV-1 infection.